Like an immune cleanup crew, natural killer cells (green) infiltrate a damaged axon. (IMAGE: ALEXANDER DAVIES / SEOUL NATIONAL UNIVERSITY AND UNIVERSITY OF OXFORD)

Scientists have known since the 1800s what happens to a totally crushed peripheral nerve in animals: the damaged axons are broken down in a process called Wallerian degeneration, allowing healthy ones to regrow. But humans rarely suffer complete axonal damage. Instead, axons tend to be partially damaged, causing neuropathic pain — a difficult-to-treat, chronic pain associated with nerve trauma, chemotherapy and diabetes.

Neuropathic pain is a hard-to-treat chronic pain condition caused by nervous system damage. For people affected, the lightest touch can be intensely painful. A study in today’s Nature may open up a new angle on treatment — and could help explain why mind-body techniques can sometimes help people manage their pain.

“We know that mental activities of the higher brain — cognition, memory, fear, anxiety — can cause you to feel more or less pain,” notes Clifford Woolf, MB, BCh, PhD, director of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital. “Now we’ve confirmed a physiological pathway that may be responsible for the extent of the pain. We have identified a volume control in the brain for pain — now we need to learn how to switch it off.” …

Neuropathic pain is chronic pain originating through some malfunction of the nervous system, often triggered by an injury. It causes hypersensitivity to innocuous stimuli and is often extremely debilitating. It doesn’t respond to existing painkillers — even opioids can’t reach it well.

New research in a mouse model, described last week in Cell Reports, deconstructed neuropathic pain and could offer new leads for treating it. The carefully done study showed that two major neuropathic pain symptoms in patients — extreme touch sensitivity and extreme cold sensitivity — operate through separate pathways.

“We think this separation will allow targeted drug-based therapies in the future,” says Michael Costigan, PhD, of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital, who was the study’s senior investigator. “If our results stand experimental scrutiny by others, this will be profoundly important in our overall understanding of neuropathic pain.” …

Chronic pain, affecting tens of millions of Americans alone, is debilitating and demoralizing. It has many causes, and in the worst cases, people become “hypersensitized”—their nervous systems fire off pain signals in response to very minor triggers.

There are no good medications to calm these signals, in part because the subjectivity of pain makes it difficult to study, and in part because there haven’t been good research models. Drugs have been tested in animal models and “off the shelf” cell lines, some of them engineered to carry target molecules (such as the ion channels that trigger pain signals). Drug candidates emerging from these studies initially looked promising but haven’t panned out in clinical testing.…

Saxitoxin produced by dinoflagellates (above), algae and shellfish could help stop neuropathic pain before it starts. (fickleandfreckled/Flickr)

A cut, a bruise, a scrape…these can all cause pain that, while unpleasant, usually passes quickly. But for an estimated 3.75 million children and adults in the United States with neuropathic pain, the pain is debilitating and never goes away.

Caused by diabetes, shingles, nerve trauma, cancer and other conditions, neuropathic pain is basically a sign that someone’s nervous system has lost track of what should and shouldn’t cause pain.

There are ways to treat or control neuropathic pain, like lifestyle changes and a range of medications, but they don’t target it at its source. Boston Children’s Hospital’s Daniel Kohane, MD, PhD, wants to do just that: to go for the root of neuropathic pain, maybe even stop it before it starts. And he’s doing it with microscopic beads full of a neurotoxin found in shellfish. …